The Third Generation Partnership Project (3GPP) initiative “License Assisted Long Term Evolution (LTE)” (LAA-LTE) aims to allow LTE equipment to operate in the unlicensed 5 Gigahertz (GHz) radio spectrum. The unlicensed 5 GHz spectrum is used as an extension to the licensed spectrum. Accordingly, devices connect in the licensed spectrum (Primary Cell (PCell)) and use Carrier Aggregation (CA) to benefit from additional transmission capacity in the unlicensed spectrum (Secondary Cell (SCell)). To reduce the changes required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the PCell is simultaneously repeated in the SCell.
Today, the unlicensed 5 GHz spectrum is used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi.”
LTE Overview
LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT) spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
As illustrated in FIG. 2, in the time domain, LTE downlink transmissions are organized into radio frames of 10 milliseconds (ms), each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each OFDM symbol is approximately 71.4 microseconds (μs).
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled; that is, in each subframe, the base station transmits control information about which terminals' data are being transmitted and upon which resource blocks the data are being transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3, or 4 OFDM symbols in each subframe and the number n=1, 2, 3, or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of, e.g., the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
From LTE Release 11 onwards, the above described resource assignments can also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). For LTE Release 8 to Release 10, only the Physical Downlink Control Channel (PDCCH) is available.
The reference symbols shown in FIG. 3 are the Cell specific Reference Symbols (CRSs) and are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
Carrier Aggregation
The LTE Release 10 standard (and subsequent releases) supports bandwidths larger than 20 Megahertz (MHz). One important requirement on LTE Release 10 is to assure backward compatibility with LTE Release 8. This should also include spectrum compatibility. That would imply that an LTE Release 10 carrier that is wider than 20 MHz should appear as a number of LTE carriers to an LTE Release 8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular, for early LTE Release 10 deployments, it can be expected that there will be a smaller number of LTE Release 10-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Release 10 carrier. The straightforward way to obtain this would be by means of CA. CA implies that an LTE Release 10 terminal can receive multiple CCs, where the CCs have, or at least have the possibility to have, the same structure as a LTE Release 8 carrier. CA is illustrated in FIG. 4.
The number of aggregated CCs as well as the bandwidth of the individual CCs may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the number of CCs is different. It is important to note that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal. A terminal may, for example, support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs.
WLAN
In typical deployments of a WLAN, Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is used. This means that the channel is sensed, and only if the channel is declared as Idle, a transmission is initiated. In case the channel is declared as Busy, the transmission is essentially deferred until the channel is found Idle. When the range of several Access Points (APs) using the same frequency overlap, this means that all transmissions related to one AP might be deferred in case a transmission on the same frequency to or from another AP which is within range can be detected. Effectively, this means that if several APs are within range, they will have to share the channel in time, and the throughput for the individual APs may be severely degraded.
A general illustration of how 802.11 stations perform CSMA/CA channel access is shown in FIG. 5. IEEE 802.11 defines a Distributed Coordination Function (DCF). The DCF coordinates the use of the medium through use of CSMA/CA and timing intervals. These timing intervals are slot time, Short Inter-Frame Space (SIFS), Distributed Inter-Frame Space (DIFS), and Extended Inter-Frame Space (EIFS). SIFS and slot time are the shortest intervals and the foundation of the others.
Licensed Assisted Access (LAA) to Unlicensed Spectrum Using LTE
Up to now, the spectrum used by LTE is dedicated to LTE. This has the advantage that the LTE system does not need to care about the coexistence issue and the spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited and, therefore, cannot meet the ever increasing demand for larger throughput from applications/services. Therefore, discussions are ongoing in 3GPP to initiate a new study item on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum. Unlicensed spectrum can, by definition, be simultaneously used by multiple different technologies. Therefore, when using unlicensed spectrum, LTE would need to consider the coexistence issue with other systems such as IEEE 802.11 (Wi-Fi). Operating LTE in the same manner in unlicensed spectrum as in licensed spectrum can seriously degrade the performance of Wi-Fi as Wi-Fi will not transmit once it detects the channel is occupied.
Furthermore, one way to utilize the unlicensed spectrum reliably is to defer essential control signals and channels on a licensed carrier. That is, as shown in FIG. 6, a User Equipment device (UE) is connected to a PCell in the licensed band and one or more SCells in the unlicensed band. In the present disclosure, a SCell in an unlicensed spectrum is referred to as a License Assisted (LA) SCell.
Efforts are currently underway to specify and implement Long Term Evolution in Unlicensed Spectrum (LTE-U) and License Assisted Access Long Term Evolution (LAA-LTE) protocols that use LTE type protocol in unlicensed frequency channels that are more commonly used by Wi-Fi devices today. To promote co-existence of LTE-U and LAA-LTE with Wi-Fi, it would be desirable for the LTE-U/LAA-LTE enhanced or evolved Node B (eNB) to select a channel that will experience the least contention and interference with Wi-Fi APs and Stations (STAs). There is a need for systems and methods for doing so without the cost of including a full Wi-Fi receiver in the eNB.